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Periodate as an Oxidant for Catalytic Water Oxidation: Oxidation via Electron Transfer or O-Atom Transfer?

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Abstract

Treatment of Ir(OH)2 with sodium periodate in aqueous solution results in formation of dioxygen following the rate law v = kobs[Ir]0.65[IO4]0.5, with kobs = 1.5 × 10–3 {Ir(OH)2 = [IrCp*(Me2NHC)(OH)2], where Me2NHC = N-dimethylimidazolin-2-ylidene and Cp* = cyclopentadienyl}. In situ ESI-MS experiments in combination with DFT calculations show that [IrIII(IO3)]+ and [IrV(=O)(IO3)]+ species are present in the reaction mixture. On the basis of the presence of these species, a mechanistic pathway was calculated illustrating that water is not necessarily the source of the oxygen. A low-lying pathway exists wherein O2 production proceeds via two consecutive O-atom-transfer reactions from periodate to the catalyst. The resulting iodite ligand is further oxidized to close the catalytic cycle. The rate-determining step in this process is formation of the O–O bond. For this transition a 21.8 kcal/mol barrier was found. This value fits very well with the observed turnover frequency of 0.27 s–1. Although it is difficult to prove that this is the dominant pathway, these data clearly illustrate that one has to be very careful with interpretation of catalytic results in periodate-driven water oxidation reactions.

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